Forces and the Laws of MotionSection 3 Newton’s First Law  Experimentation led Galileo to the idea that objects maintain their state of motion or rest.  Newton developed the idea further, in what is now known as Newton’s first law of motion:

Forces and the Laws of MotionSection 3 Newton’s First Law  Called the law of inertia  Inertia Tendency of an object not to accelerate Mass is a measure of inertia  More mass produces more resistance to a change in velocity  Which object in each pair has more inertia? A baseball at rest or a tennis ball at rest  Answer: the baseball A tennis ball moving at 125 mi/h or a baseball at rest  Answer: the baseball

Forces and the Laws of MotionSection 3 Objects at rest stay at rest and objects in motion stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. (also called the law of inertia). Inertia: the tendency of an object to resist acceleration Inertia is not a force, it’s a property of matter. More mass, more inertia

Forces and the Laws of MotionSection 3 The net force equals mass times acceleration. F net = ma or a = F net /m Explains the relationship between Net force, mass and acceleration.

Forces and the Laws of MotionSection 3 For every action force, there is an equal and opposite reaction force. Forces act in pairs!

Forces and the Laws of MotionSection 3 Objects at rest tend to remain at rest. 3.4 Newton’s Law of Inertia

Forces and the Laws of MotionSection Newton’s Law of Inertia

Forces and the Laws of MotionSection Newton’s Law of Inertia

Forces and the Laws of MotionSection 3 Newton’s Second Law  Increasing the force will increase the acceleration. Which produces a greater acceleration on a 3-kg model airplane, a force of 5 N or a force of 7 N?  Answer: the 7 N force  Increasing the mass will decrease the acceleration. A force of 5 N is exerted on two model airplanes, one with a mass of 3 kg and one with a mass of 4 kg. Which has a greater acceleration?  Answer: the 3 kg airplane

Forces and the Laws of MotionSection 3 Newton’s Second Law  F represents the vector sum of all forces acting on an object. F = F net Units for force: mass units (kg)  acceleration units (m/s 2 ) The units kgm/s 2 are also called newtons (N).

Forces and the Laws of MotionSection 3 Newton’s Third Law  Forces always exist in pairs. You push down on the chair, the chair pushes up on you Called the action force and reaction force Occur simultaneously so either force is the action force

Forces and the Laws of MotionSection 3 Newton’s Third Law  For every action force there is an equal and opposite reaction force.  The forces act on different objects. Therefore, they do not balance or cancel each other. The motion of each object depends on the net force on that object.